The present invention relates to driving circuits for a rotor position sensorless, i.e. not having a rotor position sensor, brushless DC motor, more particularly to driving methods and driving circuits for a brushless DC motor which enable high speed functions of the DC motor to be attained in a DC motor-used headphone-type stereophonic product, by enhancing the maximum speed of a rotor position sensorless brushless DC motor.
In general, a rotor position sensorless brushless three-phase DC motor can be minimized in its size because it has no rotor position sensor. According to this, its application to a light, thin and simplified product such as a headphone-type stereophonic product or the like is very useful.
In the meantime, since it is a general trend to use a single dry cell of 1.5 voltages as a power supply in a product such as a headphone-type stereophonic product or the like, the driving method for the rotor position sensorless brushless three-phase DC motor used in the product should be also active under a low voltage.
Thus, the unipolar driving method which allows only a one-directional current flow in the motor coils can not be employed because of the problems caused by the integration of PNP power transistors and a voltage drop between the collector and emitter of the power transistor.
FIG. 1 is waveforms showing the relationship between induced voltages which are back electromotive forces in accordance with phases of the brushless DC motor and turn-on time periods, having laid open in Japanese laid-open patent publication No. 58-172994.
As shown in FIG. 1, a phase difference of an electric angle 2/3.pi. exists among the induced voltages Eu, Ev, Ew in accordance with the respective phases.
That is, a 120.degree. phase difference exists as shown below in equations (1), (2) and (3). ##EQU1##
When the motor rotates in a sequential order of u, v and w phases, 60.degree. before and 60.degree. after the moment that each of the phases generates the maximum torque, i.e. 120.degree., become one of the turn-on time periods Tu, Tv, Tw of the respective phases.
The turn-on time periods Tu, Tv, Tw of the respective phases can be expressed as in equations (4), (5) and (6). ##EQU2##
FIG. 2 shows a conventional brushless DC motor driving circuit applied to the driving method of FIG. 1.
As shown in FIG. 2, it is constructed by coils 1u, 1v, 1w correspondent with phases (hereinafter, referred to as phase-corresponding coils), resistors R1, R2 for extracting a rotor position information by using back electromotive forces of the phase-correspondign coils 1u, 1v, 1w, comparators 3u, 3v, 3w for determining which of the phase-corresponding coils 1u, 1v, 1w is selected, power transistors Q1, Q2, Q3 for making a current flow through the selected one of the phase-corresponding coils 1u, 1v, 1w, and transistors Q4, Q5, Q6 for turning off by means of the turn-on signal of a next coil one of the phase-corresponding coils 1u, 1v, 1w which is active.
The operation of the conventional circuit will be described below with reference to the waveforms of FIG. 1.
Referring to FIG. 1, a high potential signal output from the comparator 3u at time t1 makes the transistor Q1 turn on so that an electric current flows through a coil 1u for the u phase (hereinafter, referred to as a u-phase coil). That is, at this time, the transistor Q4 is forced to be turned off because the transistor Q2 is turned off and the transistor Q7 is turned on.
Accordingly, at this time, since the power supply V.sub.s is applied to the non-inverting input terminal of a comparator 3u and a bit lower voltage than the power supply Vs is applied to the inverting input terminal of the comparator 3u, the output of the comparator 3u becomes high potential. This high potential signal causes a transistor Q1 to be turned on so that an electric current flows through the u-phase coil 1u. And also, at this time, since the turn-on of the transistor Q1 makes a transistor Q9 turn off and thus a transistor Q6 is turned on, the low potential signal applied to the non-inverting input terminal of the comparator 3w makes its output low potential.
Since the low potential signal turns the transistor Q3 off so that no electric current flows through a coil 1w for the w phase (hereinafter, referred to as a w-phase coil) and also the turn-off of said transistor Q3 makes a transistor Q8 turn on so that a transistor Q5 is turned off, the power supply Vs is applied to the non-inverting input terminal of the comparator 3v.
However, at this time, since the voltage of a connection point v applied to the non-inverting input terminal of the comparator 3v becomes higher than the power supply Vs and thus the output of the comparator 3v appears low potential, a transistor Q2 is turned off.
According to such operations as mentioned above, when the time t2 of FIG. 1 is met with a 120.degree. rotation of the rotor, since the voltage at the connection point v becomes a bit lower than the power supply Vs and thus the output of the comparator 3v becomes high potential, the transistor Q2 is turned on so that an electric current flows through a coil 1v for the v phase (hereinafter, referred to as v-phase coil). At this time, the turn-on of the transistor Q2 makes the transistor Q7 turn off so that the transistor Q4 is turned on, causing the output of the comparator 3u to be low potential.
Since the low potential signal makes the transistor Q1 turn off, no electric current flows through the u-phase coil 1u.
And also, at this time, since the turn-off of the transistor Q1 leads a transistor Q9 to a turn-on state, the transistor Q6 is turned off so that the power supply Vs is applied to the non-inverting input terminal of the comparator 3w.
At this time, since the voltage at a connection point W applied to the inverting input terminal of the comparator 3w becomes higher than the power supply Vs, the output of the comparator 3w becomes low potential.
The low potential signal causes the transistor Q3 to be turned off so that no electric current flows through the w-phase coil 1w.
According to such operations as mentioned above, when time t3 of FIG. 1 is met with a 120.degree. rotation of the rotor, since the state of one of the circuits for the respective phases, as described above, moves to an adjacent phase, an electric current flows through the w-phase coil 1w while stopping of an electric current flow occurs in the v-phase coil 1v and no electric current flow is kept in the u-phase coil 1u.
According to such operations as mentioned above, when time t4 of FIG. 1 is met with another 120.degree. rotation of the rotor, the procedures since the time t1 as described above are iterated.
According to this, rotational magnetic fields are generated in a sequential order of u, v and w phases while sequentially satisfying said equations 4, 5 and 6.
However, in the above-mentioned conventional circuit, since the motor makes a turn-on of only 120.degree. of the electric angle 180.degree. which generates the positive torque, torque ripples occur to prevent the rotations of the motor from being smooth so that the entire torque decreases.
And also, the maximum speed of the motor is limited to the Wmax speed at the maximum voltage.
Accordingly, the conventional circuit has a drawback in that high speed functions such as CUE and REVIEW are impossible at all and also fast forward FF and rewind REW speeds are limited by the Wmax speed and a load of the motor in case that it is applied to a product such as a headphone-type stereophonic product or the like.
The conventional rotor position sensorless brushless DC motor detects the speed of a DC motor by means of a frequency generating signal generated in a frequency generating FG coil. For this, the conventional rotor position sensorles brushless DC motor is attached with a separate frequency generating coil and also a magnet for exclusively generating a frequency or a magnet for generating a torque in an electromagnetic coil in order to induce a voltage in said frequency generating coil according to the rotation of the DC motor.
Since a separate frequency generating coil is used in order to detect a speed of the DC motor in the speed detection method for the above-mentioned conventional DC motor, the conventional circuit has another drawback in that a difficulty in manufacturing and making smaller a DC motor takes place and a precise speed detection of the DC motor is not obtained.